1. Field of the Invention
The present invention relates to a focusing technique for use in an image pick-up apparatus.
2. Description of the Related Art
Some image pick-up apparatuses, such as video cameras, use an auto focus control achieved by detecting a focus signal representing the sharpness of a screen from an image signal obtained by photoelectrically converting an object image by use of an image pick-up element, such as a charge-coupled device (CCD) or a complementary mental-oxide semiconductor device (CMOS). This control is an AF system that controls the position of a focusing lens so as to maximize the level of a focus signal (hereinafter referred to sometimes as a TV-AF process).
A focus signal used in the TV-AF process generally employs a radio-frequency component of an image signal extracted by a band-pass filter for a certain band. For a normal object image, as illustrated in a graph of FIG. 2, where the horizontal axis represents the position of a focusing lens and the vertical axis represents the level of a focus signal, the level of the focus signal increases with focusing. The point at which the focus signal exhibits the maximum level is considered as a focus position.
One of the other AF systems is a through-the-lens (TTL) phase detection process, which is widely used in film-based SLR cameras. The TTL phase detection process is achieved by first dividing a beam that has passed an exit pupil of an objective taking lens into two, then receiving the two-divided beams with a set of focus detection sensors, and thus detecting the amount of difference between signals output in accordance with the amount of received light, i.e., the amount of relative position displacement (phase difference) in directions in which the beam is divided into two. On the basis of the phase difference, the amount of deviation of the objective taking lens in a focusing direction is determined. Therefore, since the amount and direction of defocusing can be obtained by a single storing operation performed using the focus detection sensors, a high-speed focusing operation can be realized.
Another type of the phase detection is an external metering phase detection process, which has a metering sensor independently of an objective taking lens. The external metering phase detection process is achieved by receiving beams from an object with a set of focus detection sensors, then detecting the amount of relative position displacement, and thus determining an object distance from triangulation. Other examples of an AF system that uses an external metering sensor include a system that measures a propagation velocity by use of an ultrasonic sensor and a system that surveys by triangulation by use of an infrared sensor, which is often used in compact cameras.
Recently, a hybrid AF technique utilizing combination of the above described AF systems has been proposed. For example, Japanese Patent Laid-Open No. 2005-121819 and No. 5-346536 disclose a hybrid AF system that first drives a focusing lens up to the vicinity of a focus position by the TTL phase detection process and then shifts to the TV-AF process to drive the focusing lens to the focus position more precisely.
In general, a hybrid AF system first moves a focusing lens to a focus position determined by the phase detection process. Only when a focus signal exceeds a predetermined level, the phase detection process is changed to the TV-AF process, and the focusing lens is moved to a more precise focus position. This is because the TV-AF process has a higher accuracy in determining a focus state than other AF processes. However, generally, the speed of driving the focusing lens in the TV-AF process is required to be smaller than that in the phase detection process because it is necessary for the TV-AF process to detect a focus signal while moving the focusing lens.
In this respect, a technique disclosed in the first mentioned patent document (No. 2005-121819) can set an optimal TV-AF drive parameter depending on a result of comparison of focus-signal information and focus-position information obtained by a phase difference sensor. Therefore, this technique addresses a problem of focus changes leading to user discomfort by increasing accuracy in searching for the best focus position in the TV-AF process and by reducing the time required for obtaining a focus state.
Unfortunately, however, when an object is present adjacent to a camera or when a contrast of an object is low, it is often impossible to meter an object distance by use of a phase difference or, even if possible, it is difficult to obtain sufficiently accurate results by metering. The second mentioned patent document (No. 5-346536) discloses a technique for improving reliability of results of the phase detection process by determining how image patterns in the phase detection process match each other, i.e., how a pair of images formed by a focus detection optical system resemble each other, and calculating the degree of matching.
When reliability of the phase detection process is low, as in the above described examples, if output information from a phase difference sensor is used in the TV-AF process, as described in the first patent document, the speed of driving the focusing lens in the TV-AF process is sometimes significantly lower than that in the phase detection process. That is, the speed of driving the focusing lens heavily depends on an object being shot. Therefore, for a combined use of the TV-AF process and the phase detection process, the difference between the speed of driving the focusing lens in the TV-AF process and that in the phase detection process may be undesirably large depending on the state of the object.